1. Field of the Invention
This invention relates to an active matrix display including a light-emitting device.
2. Description of the Related Art
Recently, in the field of active matrix displays, attention has been paid to organic electroluminescence (EL) displays using self-luminous elements, on which research and development is conducted actively. For example, an organic EL display disclosed in Jap. Pat. Appln. KOKAI Publication No. 2007-10993 comprises a plurality of pixels arranged in a plurality of lines and rows on a substrate and forming a display screen, and a plurality of video signal lines connected to the respective pixels. Each pixel is formed of an organic EL device, which is a self-light emitting element, and a pixel circuit which supplies a driving current to the organic EL device, and performs a display operation by controlling luminous brightness of the organic EL device.
The pixel circuit includes a drive transistor formed of a thin-film transistor (hereinafter referred to as TFT), an organic EL device, a capacitor, and a switching transistor, for example. The drive transistor and the organic EL device are connected in series in this order between a high-voltage power line and a low-voltage power line. The capacitor is connected to a gate of the drive transistor. The switching transistor is connected between the video signal line, and the capacitor and the gate of the drive transistor. An output switch is provided between the drive transistor and the organic EL device.
To let an organic EL diode (hereinafter referred to as OLED) as the organic EL device emit light, a driving current according to a video signal is supplied from the drive transistor to let the OLED emit light at a brightness level determined according to the driving current.
Since OLEDs deteriorate in luminous efficiency with time approximately proportional to current density, the brightness level gradually decreases. To moderate the decrease in brightness level with time, a design approach to increase the area and decrease the current density if OLEDs has been developed.
In bottom-emission displays as organic EL displays which capture light from the substrate side on which pixels are formed, schemes to obtain light from a void between elements such as TFTs have been devised. Further, in top-emission displays which capture light to the side opposite to the substrate on which pixels are formed, schemes to eliminate limitations on arrangement of elements on a substrate upon formation of OLEDs have been devised.
For example, an OLED is arranged over a circuit element such as a TFT. A problem with this case is that a desired white brightness cannot be obtained, although the area of the OLED can be increased. Factors of this problem include increase in parasitic capacity due to overlap between a gate of a drive transistor or an electrode electrically equivalent to this gate and an anode of the OLED over a large area.
That is, increase in parasitic capacitance causes propagation of change in potential of the gate of the drive transistor over change in potential of the anode of the OLED. The amount of displacement of the anode potential of the OLED is large when an output switch is turned on. Immediately before the output switch is turned on (when writing is finished), the anode potential of the OLED is equivalent to a threshold voltage of the OLED, and a gate potential of the drive transistor is at a predetermined potential determined according to a video signal. When the output switch is turned on, a voltage is applied to the OLED and the anode potential increases, and thereby the OLED emits light.
The parasitic capacitance also increases the gate potential of the drive transistor (in an off direction) with increase in anode potential, and thereby a current supplied to the OLED decreases and the OLED will emit light with a brightness lower than a desired brightness. Further, change in anode potential of an OLED when an output switch is turned on decreases as the brightness becomes lower and increases as the brightness becomes higher. Accordingly, the parasitic capacitance will also lose a dynamic range of the video signal voltage.